Developers of information storage devices and methods continue to seek increased storage capacity. As parts of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a signal beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a signal beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image. More than one holographic image may be stored in the same volume by, for example, varying the angle of the reference beam during recording.
The capabilities of holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage medium for research purposes for many years. However, lithium niobate is expensive, exhibits poor sensitivity (1 J/cm2), has low index contrast (Δn of about 10−4), and exhibits destructive read-out (i.e., images are destroyed upon reading). Alternatives have therefore been sought, particularly in the area of photosensitive polymer films. See, e.g., W. K. Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains a precursor material containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the precursor material is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data. Unfortunately, deposition onto a substrate of the pre-formed matrix material containing the precursor material requires use of solvent, and the thickness of the material is therefore limited, e.g., to no more than about 150 μm, to allow enough evaporation of the solvent to attain a stable material and reduce void formation.
In holographic processes such as described above, which utilize three-dimensional space of a medium, the storage capacity is proportional to a medium's thickness. Thus, the need for solvent removal inhibits the storage capacity of a medium. (Holography of this type is typically referred to as volume holography because a Klein-Cook Q parameter greater than 1 is exhibited (see W. Klein and B. Cook, “Unified approach to ultrasonic light diffraction,” IEEE Transaction on Sonics and Ultrasonics, SU-14, 1967, at 123-134). In volume holography, the media thickness is generally greater than the fringe spacing,)
U.S. Pat. No. 6,013,454 and application Ser. No. 08/698,142, the disclosures of which are hereby incorporated by reference, also relates to a precursor material in an organic polymer matrix. In particular, the application discloses a recording medium formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a precursor material. A similar type of system, but which does not incorporate oligomers, is discussed in D. J. Lougnot et al., Pure and Appl. Optics, 2, 383 (1993). Because little or no solvent is typically required for deposition of these matrix materials, greater thicknesses are possible, e.g., 200 μm and above. However, while useful results are obtained by such processes, the possibility exists for reaction between the precursors to the matrix polymer and the photoactive monomer. Such reaction would reduce the refractive index contrast between the matrix and the polymerized photoactive monomer, thereby affecting to an extent the strength of the stored hologram.
Furthermore, the widespread use of holographic recording media has been limited due to low speed of manufacturing the media using conventional processes and conventional precursors. For example, as disclosed in JP-B No. 3330854, conventionally, a mixture of polymer precursor(s), solvent and photopolymerizing materials has been used as a precursor material for fabricating an organic hologram recording medium. Such a precursor material needs to be diluted with a solvent to be applied as a film/layer onto a substrate, leading to several problems such as void formation in the holographic recording medium due to evaporation of the solvent and a relatively long time to thicken the precursor material layer because such a process requires drying the solvent.
In the case of holographic recording media, since a signal recording density improves as the thickness of the recording material layer increases, an increase in recording material layer thickness is a very preferred factor. However, increasing the layer thickness causes concomitant problems stated above.
Another problem associated with convention processes for manufacturing holographic recording media is optical distortion of the holographic recording media. In particular, the processes for manufacturing holographic recording media that continuously mixes the precursor material constituents and applies the obtained mixture on the substrates using a mixer, is optical distortion of the holographic recording medium. The optical distortion is most likely to occur due to a difference in contraction with polymerization of the precursor material supplied from the mixer, resulting in adverse effects on the characteristics of the holographic recording media.
Moreover, many of conventional organic holographic recording media have a structure with the holographic recording material layer inserted between two substrates, a so-called sandwich structure. The sandwich structure is preferred because the holographic layer could be protected by the outer substrate layers. However, it is difficult to make the sandwich structure, particularly in a single manufacturing operation, by the conventional processes for manufacturing holographic recording medium using a system. In the case of the methods commonly used to apply a two-liquid mixing material, wherein a line of material is applied on the substrate by the mixer and another substrate is covered on the precursor material layer so that two substrates may sandwich the precursor material layer between themselves, adverse effects may be exerted on the shapes of light beams irradiated onto the media at hologram recording and/or reading out because the extent of material polymerization depends on the point at which the material is applied, leading to an unevenness in film thickness.
Thus, while progress has been made in fabricating photorecording media suitable for use in holographic storage systems, further progress is desirable. In particular, the urgent need exists for a reliable, high speed process for manufacturing holographic recording media and for precursors adapted for such a process.